Subroutines and Calling Conventions Solution

1.1    Purpose

Now that you’ve been introduced to assembly, think back to some high level languages you know such as Python or Java. When writing code in Python or Java, you typically use functions or methods. Functions and methods are called subroutines in assembly language.

In assembly language, how do we handle jumping around to different parts of memory to execute code from functions or methods? How do we remember where in memory the current function was called from (where to return to)? How do we pass arguments to the subroutine, and then pass the return value back to the caller?

The goal of this assignment is to introduce you to the Stack and the Calling Convention in LC-3 Assembly. This will be accomplished by writing your own subroutines, calling subroutines, and even creating subroutines that call themselves (recursion). By the end of this assignment, you should have a strong understanding of the LC-3 Calling Convention and the Stack Frame, and how subroutines are implemented in assembly language.

1.2    Task

You will implement each of the three subroutines (functions) listed below in LC-3 assembly language. Please see the detailed instructions for each subroutine on the following pages. The autograder checks for certain subroutine calls with arguments pushed in the correct order, so we suggest that you follow the provided algorithms when writing your assembly code. Your subroutines must adhere to the LC-3 calling convention.

    1. factorial.asm

    2. bubbleSort.asm

    3. binarySearch.asm

More information on the LC-3 Calling Convention can be found on Canvas under Lab slides 9 and 10 and in Lecture Slides L10 and L11. More detailed information on each LC-3 instruction can be found in the Patt/Patel book Appendix A (also on Canvas under LC3 Resources). Please check the rest of this document for some advice on debugging your assembly code, as well some general tips for successfully writing assembly code.

1.3    Criteria

Your assignment will be graded based on your ability to correctly translate the given pseudocode for sub-routines (functions) into LC-3 assembly code, following the LC-3 calling convention. Please use the LC-3 instruction set when writing these programs. Check the deliverables section for deadlines and other related information.

You must obtain the correct values for each function. In addition, registers R0-R5 and R7 must be restored from the perspective of the caller, so they contain the same values before and after the caller’s JSR call. Your subroutine must return to the correct point in the caller’s code, and the caller must find the return value on the stack where it is expected to be. If you follow the LC-3 calling convention correctly, each of these things will happen automatically.

While we will give partial credit where we can, your code must assemble with no warnings or errors. (Complx will tell you if there are any.) If your code does not assemble, we will not be able to grade that file and you will not receive any points. Each function is in a separate file, so you will not lose all points if one function does not assemble. Good luck and have fun!


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    • Detailed Instructions

2.1    Part 1

2.1.1    Factorial

The Factorial of a number is the product of that number and all of the numbers below it. In factorial.asm, we want you to implement two subroutines: MULTIPLY and FACTORIAL (see the following pseudocodes for more details). Note that the numbers that we will give you are strictly greater than or equal to 0. As a reminder, please do not change the names of provided subroutines. Otherwise your submission will not pass the autograder.

For example:

factorial(0) should return 1

factorial(1) should return 1

factorial(2) should return 2

factorial(3) should return 6



2.1.2    Pseudocode

Here are the pseudocodes for these subroutines:

MULTIPLY(int a, int b) {

int ret = 0;

while (b > 0):

ret += a;

b--;

return ret;

}

FACTORIAL(int n) {

int ret = 1;

for (int x = 2; x <= n; x++) {

ret = MULTIPLY(ret, x);

}

return ret;

}

Note: since there are two loops in the MULTIPLY and FACTORIAL subroutines, make sure you use different label names for those loops. In general, you should not have two loops with the same name in the same program.
















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2.2    Part 2

2.2.1    Bubble Sort

For this part of this assignment, you will be implementing bubble sort on an array of integers in bubbleSort.asm. As a reminder, please do not change the names of provided subroutines. Otherwise your submission will not pass the autograder.

If you are unfamiliar with bubble sort and would like more information about it, see https://www.geeksforgeeks.

org/bubble-sort/.

2.2.2    Pseudocode

Here is the pseudocode for this subroutine:

BUBBLE_SORT(int[] arr (addr), int length) {

int swapped = 0;

for (int i = 0; i < length - 1; i++) {

if (arr[i] > arr[i + 1]) {

int temp = arr[i + 1];

arr[i + 1] = arr[i];

arr[i] = temp;

swapped = 1;

}

}

if (swapped == 1) {

BUBBLE_SORT(arr, length - 1);

}

}

Let’s do an example of bubble sort in action and how the array looks after each iteration given our algorithm above!

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3
1
1
6
Given






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1
1
3
6
swapped = 1; call BUBBLE SORT(arr, 4)






1
1
2
3
6
swapped = 1; call BUBBLE SORT(arr, 3)






1
1
2
3
6
swapped = 0; no call























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2.3    Part 3

2.3.1    Binary Search

For this part of the assignment, you will write a recursive search subroutine for a binary tree in binarySearch.asm. The parameters of the function is the tree’s root (provided as an address in memory) and the data you need

to search for. Return null or 0 if the data does not exist in the tree, else return the data address if found. As a reminder, please do not change the names of provided subroutines or otherwise your submission will not pass the autograder.

2.3.2    Binary Tree Data Structure

The below figure has 3 parts (the actual tree, the memory layout of that tree and the visualization of that tree) depicting how each node in our tree is laid out in our memory. Each node will have three attributes: its value, its left node, and its right node. Note that each node is treated as an address in memory.

Given some node that lives at address x:

    • mem[x] = node.data

    • mem[x + 1] = node.left

    • mem[x + 2] = node.right

For example, x4000 holds the data of a node, x4001 holds the address of the left child of that node, and x4002 holds the address of the right child of that node. For a leaf node (a node with no child) such as node 8 in our example, x4008 holds the data and x4009 and x400A both hold 0 since node 8 has no child nodes.


























See https://www.geeksforgeeks.org/binary-tree-data-structure/ for more information regarding bi-nary trees.

2.3.3    Pseudocode

Here is the pseudocode for this subroutine:


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BINARY_SEARCH(Node root (addr), int data) {

if (root == 0) {

return 0;

}

if (data == root.data) {

return root;

}

if (data < root.data) {

return BINARY_SEARCH(root.left, data);

}

return BINARY_SEARCH(root.right, data);

}

*Note: 0 is equivalent to NULL


















































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    • Autograder

To run the autograder locally, follow the steps below depending upon your operating system:

    • Mac/Linux Users:

        1. Navigate to the directory your homework is in (in your terminal on your host machine, not in the Docker container via your browser)

        2. Run the command sudo chmod +x grade.sh

        3. Now run ./grade.sh

    • Windows Users:

        1. In Git Bash (or Docker Quickstart Terminal for legacy Docker installations), navigate to the directory your homework is in

        2. Run chmod +x grade.sh

        3. Run ./grade.sh

Note: The checker may not reflect your final grade on this assignment. We reserve the right to update the autograder as we see fit when grading.


    • Deliverables

Turn in the following files to Gradescope:

    1. factorial.asm

    2. bubbleSort.asm

    3. binarySearch.asm

Note: Please do not wait until the last minute to run/test your homework. Last minute turn-ins will result in long queue times for grading on Gradescope. You have been warned.

























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    • Demos

This homework will be demoed. The demos will be ten minutes long and will occur IN PERSON.

Stay tuned for details as the due date approaches.

    • Sign up for a demo time slot via Canvas before the beginning of the first demo slot. This is the only way you can ensure you will have a slot.

    • If you cannot attend any of the predetermined demo time slots, e-mail the head TA Shawn Wahi (shawn.wahi@gatech.edu) before the week of demos.

    • If you know you are going to miss your demo, you may cancel your slot on Canvas with no penalty, as long as you cancel 24 hours in advance. However, you are not guaranteed another time slot. If you cancel within 24 hours of your demo, it will be counted as a missed demo.

    • Your overall homework score will be ((homework_score * 0.5) + (demo_score * 0.5)), meaning if you received a 90% on your homework, but a 30% on the demo, you would receive an overall score of 60%. If you miss your demo you will not receive any of these points, and the maximum you can receive on the homework is 50%.

    • You will be able to make up one of your missed demos at the end of the semester for half credit.










































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    • Appendix

6.1    Appendix A: LC-3 Instruction Set Architecture































































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    • Debugging LC-3 Assembly

When you turn in your files on Gradescope for the first time, you may not receive a perfect score. Does this mean you change one line and spam Gradescope until you get a 100? No! You can use a handy Complx feature called “replay strings”.

    1. First off, we can get these replay strings in two places: the local grader, or off of Gradescope. When you run the autograder, you should see an output like this:
















Copy the string, starting with the leading ’B’ and ending with the final backslash. Do not include the quotation marks.

Side Note: If you do not have Docker installed, you can still use the tester strings to debug your assembly code. In your Gradescope error output, you will see a tester string. When copying, make sure you copy from the first letter to the final backslash and again, don’t copy the quotations.













2. Secondly, navigate to the clipboard in your Docker image and paste in the string.























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3. Next, go to the Test Tab and click Setup Replay String










4. Now, paste your tester string in the box!

















    5. Now, Complx is set up with the test that you failed! The nicest part of Complx is the ability to step through each instruction and see how they change register values. To do so, click the step button. To change the number representation of the registers, double click inside the register box.








    6. If you are interested in looking how your code changes different portions of memory, click the view tab and indicate ’New View’



















    7. Now in your new view, go to the area of memory where your data is stored by CTRL+G and insert the address



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    8. One final tip: to automatically shrink your view down to only those parts of memory that you care about (instructions and data), you can use View Tab → Hide Addresses → Show Only Code/Data.














































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7.1    Appendix C: LC-3 Assembly Programming Requirements and Tips

    1. Your code must assemble with NO WARNINGS OR ERRORS. To assemble your program, open the file with Complx. It will complain if there are any issues. If your code does not assemble you WILL get a zero for that file.

    2. Comment your code! This is especially important in assembly, because it’s much harder to interpret what is happening later, and you’ll be glad you left yourself notes on what certain instructions are contributing to the code. Comment things like what registers are being used for and what less intuitive lines of code are actually doing. To comment code in LC-3 assembly just type a semicolon (;), and the rest of that line will be a comment.

    3. Avoid stating the obvious in your comments, it doesn’t help in understanding what the code is doing.

Good Comment

ADD R3, R3, -1    ; counter--

BRp LOOP    ; if counter == 0 don’t loop again

Bad Comment

ADD R3, R3, -1    ; Decrement R3

BRp LOOP    ; Branch to LOOP if positive

    4. DO NOT assume that ANYTHING in the LC-3 is already zero. Treat the machine as if your program was loaded into a machine with random values stored in the memory and register file.

    5. Following from 3. You can randomize the memory and load your program by doing File - Randomize and Load.

    6. Use the LC-3 calling convention. This means that all local variables, frame pointer, etc. . . must be pushed onto the stack. Our autograder will be checking for correct stack setup.

    7. Start the stack at xF000. The stack pointer always points to the last used stack location. This means you will allocate space first, then store onto the stack pointer.

    8. Do NOT execute any data as if it were an instruction (meaning you should put .fills afterHALT or RET).

    9. Do not add any comments beginning with @plugin or change any comments of this kind.

    10. Test your assembly. Don’t just assume it works and turn it in.



















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    • Appendix D: Rules and Regulations

8.1    General Rules

    1. Although you may ask TAs for clarification, you are ultimately responsible for what you submit. As such, please start assignments early, and ask for help early. This means that (in the case of demos) you should come prepared to explain to the TA how any piece of code you submitted works, even if you copied it from the book or read about it on the internet.

    2. If you find any problems with the assignment it would be greatly appreciated if you reported them to the author (which can be found at the top of the assignment). Announcements will be posted if the assignment changes.

8.2    Submission Conventions

    1. Do not submit links to files. The autograder does not understand it, and we will not manually grade assignments submitted this way as it is easy to change the files after the submission period ends. You must submit all files listed in theDeliverables section individually to Gradescope as separate files.

8.3    Submission Guidelines

    1. You are responsible for turning in assignments on time. This includes allowing for unforeseen circum-stances. If you have an emergency let us know IN ADVANCE of the due time supplying documenta-tion (i.e. note from the dean, doctor’s note, etc). Extensions will only be granted to those who contact us in advance of the deadline and no extensions will be made after the due date.

    2. You are also responsible for ensuring that what you turned in is what you meant to turn in. After submitting you should be sure to download your submission into a brand new folder and test if it works. No excuses if you submit the wrong files, what you turn in is what we grade. In addition, your assignment must be turned in via Canvas/Gradescope. Under no circumstances whatsoever we will accept any email submission of an assignment. Note: if you were granted an extension you will still turn in the assignment over Canvas/Gradescope.

8.4    Syllabus Excerpt on Academic Misconduct

Academic misconduct is taken very seriously in this class. Quizzes, timed labs and the final examination are individual work.

Homework assignments are collaborative, In addition many if not all homework assignments will be evaluated via demo or code review. During this evaluation, you will be expected to be able to explain every aspect of your submission. Homework assignments will also be examined using computer programs to find evidence of unauthorized collaboration.

What is unauthorized collaboration? Each individual programming assignment should be coded by you. You may work with others, but each student should be turning in their own version of the assignment. Submissions that are essentially identical will receive a zero and will be sent to the Dean of Students’ Office of Academic Integrity. Submissions that are copies that have been superficially modified to conceal that they are copies are also considered unauthorized collaboration.

You are expressly forbidden to supply a copy of your homework to another student via elec-tronic means. This includes simply e-mailing it to them so they can look at it. If you supply an electronic copy of your homework to another student and they are charged with copying, you will also be charged. This includes storing your code on any site which would allow other parties to obtain your code such as but not limited to public repositories (Github), pastebin, etc. If you would like to use version control, use github.gatech.edu


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8.5    Is collaboration allowed?

Collaboration is allowed on a high level, meaning that you may discuss design points and concepts relevant to the homework with your peers, share algorithms and pseudo-code, as well as help each other debug code. What you shouldn’t be doing, however, is pair programming where you collaborate with each other on a single instance of the code. Furthermore, sending an electronic copy of your homework to another student for them to look at and figure out what is wrong with their code is not an acceptable way to help them, because it is frequently the case that the recipient will simply modify the code and submit it as their own.


























































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